The present invention concerns a fixed spike, or a sleeve-mounted spike, fitted on a vehicle tire.
Increasing traffic load and spike tires in combination have proved to be a remarkable road attrition factor. In some countries this has even led to prohibition of spike tires, or at least to considerable restrictions.
In Nordic conditions, the beneficial effect of anti-slip means on the safety and flexibility of traffic has on the other hand been irrefutably demonstrated, and this effect should not be sacrificed: instead, the drawbacks should merely be eliminated. Good results will be achieved by further developing both the road super-structures and the anti-slip tires.
As a pneumatic automobile tire rolls on an even surface, it is considerably flattened radially, owing to its flexibility, whereby in the contact region longitudinal as well as transversal forces are generated owing to changes of the rolling radius.
The longitudinal forces acting on the spike when the tire is rolling are due to bending of the body structure, to longitudinal slipping and to the stress wave in the rubber. The rubber surface pattern of the tire carries the load so that its internal stress state increases. The rubber material is not compressed, but due to its elasticity it changes shape.
When a spike approaches the point of contact with the road, the tire body undergoes bending such that the radius of the bent part is significantly smaller than that of equivalent parts of the load-free tire. This deflects the spike, which has been mounted at right angles against the surface, to assume a vertical position before the contact with the road. Owing to protrusion of the spike tip, however, the spike is not turned into sufficiently upright position: it meets the road surface in an oblique position. At this stage, the forces due to slipping tendency also begin to exert their influence.
As the rotating tire surface and the protruding fixed body (the spike) attached thereto meet the road surface, the spike is pushed into the rubber material on a length dependent on the hardness of the base; on a hard road super-structure the spike is pressed entirely into the rubber material of the tire, while on softer ice and snow surfaces the tip part of the spike remains outside the wear surface of the tire and causes increase of friction. At the same time, the wear surface pattern composed of rubber material is compressed between the body structure of the tire and the road surface, whereby the rubber material is not compressed, but owing to its elasticity it flows and assumes, as it comes under strain, a configuration different from its original shape. The flowing and deformation of the rubber are determined by the load, the rubber material, the dimensions and configuration of the wear surface, and such solid bodies as are embedded in the rubber.
When the rubber is compressed between the forces acting from opposite directions, the deformation is different in different parts of the wear surface pattern. Since the rubber is firmly fixed to the body structure of the tire and the friction against the road surface prevents any horizontal movement of the wear surface, the rubber is forced to flow towards the free surfaces of the wear surface pattern. The vertical displacements are due to flowing of the rubber under the flange of the spike outwards from the central axis, and to the shape of the spike body and the elastic yielding caused by the load. Since the rubber is not compressed, the vertically directed pressure is converted into displacements in the horizontal plane, as rubber molecules flow past each other towards free surfaces, which bulge outwards as a result. Owing to the configuration of the wear surface pattern and the stress caused by the spike, there remains a neutral region within the pattern element, around the spike, where the movement of the rubber is minimal or there is none at all, unless such movement is caused by the shape of the spike.
Traditionally, in designing anti-slip means the rubber flow is not controlled to support the holding properties of the spike, in addition to which such means of prior art have damaged the rubber material or the body structures of the tire. The shapes have featured sharp edges or detrimental discontinuities, which fray the bonds in the rubber as the rubber material flows past them and owing to the stress state is urged against them, with the result that the spike gets loose and its holding properties deteriorate because of reduced support from the rubber. Detrimental configurations have also been the shapes having two or more flanges in which the rubber flow is improperly prevented so that the rubber material locked between the flange ridges is torn off as a result of excessive shearing stress.
When the spike shape is disadvantageous regarding rubber flow, the slipping of the rubber in particular in the region of the body part of the spike may be so great that it causes strong abrasion of the spike and of the rubber.
In spikes of prior art, the shape of the body is such in some instances that when the spike is pressed into the rubber and the rubber flows into the sharp-bottomed grooves of the body, the rubber simultaneously slides slightly along the spike body towards the flange and causes the spike to rise towards the wear surface of the tyre when the spike returns to its position at rest on termination of road contact. These differentially small slips of the rubber distinctly increase the protrusion of the spike at the initial phase of using such an incorrectly designed spike.
Discontinuities in the shape of the spike in which closed air pockets are formed are likewise detrimental. When the spike is pressed into the tire and the rubber flows into such air pockets, the pressure of the rubber is significantly higher than the air pressure, whereby the air is compressed at each revolution of the tire in rapid impact and is powerfully heated and causes damage to the surrounding rubber.
In conventional spike designs attempts have been made to prevent rotation of the spike around its axis, or no active attempts have been made to do this in any design of prior art. This has resulted in uneven wear of the whole spike, particularly in uneven wear of its hard-metal tip, which in turn has led to weakening of the holding characteristics of the tire during use.